Removing metallic oxides from metallic surfaces, otherwise known as pickling, is necessary prior to coating a metal, such as steel, magnesium and magnesium alloys, aluminum and aluminum alloys, zinc and zinc alloys, copper and copper alloys, etc., with any kind of finish, including, for example, electroplating, electroless plating, immersion plating, paint or conversion coating.
Historically, strong acids have been used as pickling agents, including hydrochloric acid, sulfuric acid, nitric acid and phosphoric acid, with a typical pH in the range of about 0.5 to 3.0. Hydrochloric acid and nitric acid yield the best pickled surface but are corrosive to the surrounding equipment and installation. Sulfuric acid and phosphoric acid are not volatile, but their ferrous salts are not as soluble as ferrous chloride and nitrates, and the resulting pickled surface may be rather blemished, which can affect the coating appearance.
Among all metallic oxides, including rust, heat treated scale on steel is the most challenging for removal. Iron oxides formed during heat treatment, including FeO, Fe2O3 and Fe3O4 (magnetite), have different solubilities in acids and are layered. FeO, which is the most soluble, constitutes the first layer next to the base metal, and magnetite, which is the least soluble, constitutes the outer layer. Typically, heat affected areas on steel are cracked due to cooling after welding or annealing. The pickling acids work by infiltrating the upper layer through the cracks, quickly dissolving the bottom layer FeO by protonation.
The base metal, Fe(s) is oxidized by H+ that is reduced to H2(g). As a result, small electrolytic cells are created where the exposed steel, Fe, is the anode, the acid is the electrolyte and the upper layer, magnetite Fe3O4 is the cathode. The nascent H2(g) reduces the magnetite into ferrous ions, that are soluble according to the following equation:Fe3O4+H2(g)+6H+43Fe+++4H2O  (1)
Magnetite is dissolved by a redox reaction at a slower rate than the other oxides. It is also magnetic and difficult to shed. Depending on furnace conditions and cycles, the magnetite layer can be thick and tightly uniform and adherent, which can create acid-resistant scale that requires mechanical scale cracking such as shot blasting or roll bending to loosen the scale prior to acid pickling, as described, for example in U.S. Pat. No. 5,743,968 to Leeker et al. and U.S. Pat. No. 5,879,465 to McKevitt et al., the subject matter of each of which is herein incorporated by reference in its entirety. The addition of fluoride to the acid pickling composition has been found to help crack the scale.
If the magnetite layer is uneven, longer immersion times can be required for removal of the magnetite layer. This is problematic because over-pickling can form spots and smut (especially with sulfuric acid), compromising the coating appearance. Long immersion times in acid can also create pitting, where the acid is trapped, leading to delayed blisters under the coating or simply unacceptable appearance. Finally, the H2(g) produced by reaction between acid and Fe(s), adsorbs onto, and penetrates into the steel surface, creating hydrogen embrittlement and causing mechanical failure in the field especially with hardened steel. Pertinent industry specifications limit immersion times in acid baths to a maximum of 10 minutes to avoid hydrogen embrittlement on hardened steel from the pickling step.
Mechanical descaling can be used but is expensive and cannot clean internal surfaces of tubular steel. Media blasting and vibratory finishing are time consuming and costly, although they still are widely used to remove heat treated scale, although they may provide insufficient cleaning of tubular parts and recessed surfaces. Pickling in strong acids is problematic in cast iron because of pores in the cast iron that can trap acids. Equally challenging are amphoteric metals such as zinc and aluminum, which have an oxide layer that should be removed before coating. However, the base metal may be severely attacked in acidic or alkaline solutions.
Concerns from pickling in hydrochloric and nitric acids have driven the industry towards the use of non-fuming acids, weak organic acids and neutral pickles. Because protonation of oxides, by H+ from acids is not enough and a redox reaction is needed, many oxidizer-containing processes have been created to oxidize the base metal iron, copper, tin and zinc, in order to remove superficial scale. These oxidizers, such as nitric acid, hydrogen peroxide, permanganate, persulfate and nitro compounds, are combined with acids, H+, or complexing agents that dissolve metallic oxides. A corrosion inhibitor is added, to prevent rapid atmospheric oxidation at the exit of the pickling solution. Nitric acid and hydrogen peroxide, in particular, foul the rinses quickly with ferric ions and foster flash rusting of the surface in a matter of seconds. These combinations have served the metal industry under different functions: including pickling, as described, for example, in U.S. Pat. No. 6,500,328 to Fortunati et al., descaling, as described, for example, in U.S. Pat. No. 5,377,398 to Bessey, polishing, as described, for example, in U.S. Pat. No. 6,750,128 to Kondo et al., and stripping steel and other metals as described, for example, in U.S. Pat. No. 4,687,545 to Williams et al. and U.S. Pat. No. 4,720,332 to Coffey, the subject matter of each of which is herein incorporated by reference in its entirety.
When the oxidizer is m-nitrobenzene sulfonic acid, or one of its salts, and is combined with organic phosphonates, the processes are either acidic, as described, for example, in U.S. Pat. No. 6,407,047 to Mehta et al., or alkaline (i.e., pH of about 6-14) as described, for example, in U.S. Pat. No. 4,042,451 to Lash, the subject matter of each of which are herein incorporated by reference in its entirety. These oxidizers can be used as metal strippers. However they tend to leave a dark adherent film on the surface, requiring subsequent cleaning and pickling.
When the pH is neutral, and the purpose is descaling steel as described, for example, in U.S. Pat. No. 8,323,416 to Bradley, the subject matter of which is herein incorporated by reference in its entirety, the chemicals used may be completely different from those described in the current invention. For example, U.S. Pat. No. 4,437,898 to Drosdziok, the subject matter of which is herein incorporated by reference in its entirety, describes a passivation process that imparts corrosion inhibition to steel surfaces. This is a weakly alkaline process with a pH between 7.5-10.5, with organic phosphonates but does not contain an oxidizer, let alone a nitro compound, that is capable of removing heat treated scale.
U.S. Pat. No. 7,344,602 to Varrin et al., the subject matter of which is herein incorporated by reference in its entirety, describes a magnetite scale removal process with a pH neutral chemical solution containing complexing agents to soften the scale assisted with a hydro-mechanical cleaning to completely remove the scale. U.S. Pat. No. 7,396,417 to Fischer et al., the subject matter of which is herein incorporated by reference in its entirety, describes an aqueous pickling solution with carboxylic acids that operates at a pH between 2.5-4.0 but does not contain nitro compounds nor phosphonates.
None of the known prior art processes describe an aqueous pickling process that operates at a near neutral pH, provides an improved scale removal mechanism, improves corrosion inhibition and that has a mild attack of substrate.
There also remains a need in the art for an improved pickling composition that is capable of removing metallic oxides, including magnetite and other problematic metallic oxides in an efficient manner and that can operate at a near neutral pH and at ambient temperature.